Recently, the portable terminals such as laptop computer, mobile phone and PDA are widely used. For secondary batteries used for power sources of these portable terminals, nickel hydrogen secondary battery, lithium ion secondary battery or so are heavily used. The portable terminals are rapidly downsized, made thinner, have become lighter, and has higher performance. As a result, the portable terminals are used in various occasions.
Also, for the battery, it is required to be smaller, thinner, and lighter and to have higher performance as similar to the portable terminals. Also, in order to increase the blending amount of the active material in the active material layer in order to have higher capacity for the battery, it is demanded to reduce the material such as the binder for fixing the active material on the current collector, or the conductive material for securing the conductivity.
In order to have higher capacity for the lithium ion secondary battery, the lithium ion secondary battery negative electrode using the alloy based active material comprising Si or so was is developed (for example, the patent document 1). Along with having higher capacity, the voltage during the charging-discharging are becoming higher, and for the electrolytic solution made of ethylene carbonate or propylene carbonate or so, it cannot withstand the high voltage and may decompose, thus the fluorine based electrolytic solution additives are used together.
On the other hand, for the positive electrode, as for the binder, the fluorine containing polymer such as polyvinylidene fluoride (PVdF) or so has been used. Fluorine containing polymer does not dissolve to the electrolytic solution, thus the stable binding property can be expected, however the fluorine containing polymer such as PVdF is hard and it is difficult to bend. Therefore, depending on the shape or the size of the battery, when only using the fluorine containing polymer in case of molding into a predetermined shape by pressing after winding the electrode, the cracks may be formed to the active material layer.
Hence, as the binder used for the positive electrode, it's being considered to together use the fluorine containing polymer and nitrile rubber (for example, the patent document 2), or the fluorine containing polymer and crosslinked acrylate based polymer (for example, the patent document 3)